The present invention generally relates to liquid metering pumps for delivering controlled amounts of liquid from one vessel to another, or from a source of supply to a process stream. More particularly, it relates to a new and improved automatic venting back pressure valve for use on a liquid metering pump to provide a desirable back pressure to ensure that the metering pump meters fluids accurately.
Diaphragm metering pumps are known and used for transferring fluids from one place to another. Generally, diaphragm pumps include a pumping head area including a product chamber bounded on one side by a displacable diaphragm member. The inlet and exit to the product chamber are provided with one way check valves. As the diaphragm is displaced away from the product chamber, the exit check valve closes under reduced pressure, the inlet check valve opens and fluid is drawn into the product chamber. Thereafter, as the diaphragm is displaced toward the product side, pressure increases on the fluid in the product chamber, closing the inlet check valve, opening the outlet check valve and forcing fluid in the product chamber out of the exit. In continuous operation, a diaphragm pump pumps fluid through the product side in a pulsed manner.
Diaphragm displacements may be achieved with a mechanical drive system or an hydraulic drive system. An example of the mechanical drive is a solenoid pump. In a solenoid pump, a diaphragm actuator rod is secured at one end to the diaphragm and at its opposed end is connected to a solenoid actuator. The electrically or electronically controlled solenoid is effective to cause reciprocal linear movement of the actuator and diaphragm actuator rod thereby causing displacements of the diaphragm directly.
In an hydraulically driven diaphragm metering pump, diaphragm displacement is achieved by varying the pressure of a hydraulic fluid on the hydraulic side of the diaphragm through operation of a reciprocating piston disposed in fluid communication with a hydraulic chamber. Instead of direct mechanical attachment to the diaphragm, with this type of pump, a hydraulic fluid is pressurized on one side of the diaphragm to cause diaphragm displacements toward or away from the product chamber. This also results in a pulsed pumping of a fluid through the pump head.
Excellent examples of these types of diaphragm metering pumps are described in commonly assigned U.S. Pat. No. 5,667,368 which issued Sep. 16, 1997 and U.S. Pat. No. 5,676,531 which issued Oct. 14, 1997. The disclosures of each of these prior patents are specifically incorporated herein by reference.
All diaphragm metering pumps require some back pressure to meter fluids accurately. This pressure may range from about 5 to 35 psi, preferably from about 10 to about 25 psi. Back pressure is necessary to ensure proper check valve re-seating for the product outlet or discharge ball check valve. A lack of back pressure causes or permits fluid flow through in excess of theoretical flow. The use of a back pressure valve assists in providing required back pressure and is especially useful when pumping fluids into a lower or no pressure receptacle or system in order to maintain metering accuracy.
A problem which may arise in diaphragm metering pumps occurs during operation if a volume of air is sucked into the intake lines so that air travels through the suction line, or after sitting idle, gas accumulates in the pump head. Air or gas in the intake or pump head may cause the pump to lose prime. If the pump loses its prime and gas fills the diaphragm metering pump head area, because of the compressability of gas, pumping displacements of the diaphragm may simply compress the gas and not result in any liquid pumping or fluid flow. If there is a loss of priming, frequently a pump can not regain hydraulic firmness and restart pumping.
Accordingly, it is an object of the present invention to provide a new and improved automatic venting back pressure valve which provides a gas by-pass feature allowing trapped air or gas to be pumped and moved around a back pressure restriction device so that, in the event air is introduced into the pumping chamber, the air can be rapidly removed and the pump can self prime to resume pumping liquid.